Mitochondria! myopathy and sideroblastic anemia (MLASA) is a rare autosomal recessive disease with oxidative phosphorylation abnormalities. Genetic mapping in affected families localized the disease gene within 1.2 Mb of chromosome 12q24.33, and sequence analysis revealed a homozygous missense mutation in the pseudouridine synthase 1 gene in all patients with MLASA in these families. Biochemical characterization revealed an absence of pseudouridylation at the expected sites in mitochondrial and cytoplasmic tRNAs of patients. The broad long term goals of this research project are (1) to understand the role of pseudouridylation in human disease; (2) to elucidate the pathophysiological pathways of the molecular defect to clarify the tissue specificity and variability of disease expression in MLASA; (3) to provide the basis for the development of diagnostic, preventive, and therapeutic interventions for these patients; and (4) to use this simple Mendelian model of a tissue specific oxidative phosphorylation disease to potentially shed light on the clinical expression of mitochondrial DNA disorders. The specific aims of this three year proposal are (1) to characterize the expression patterns and splice variants of PUS1 qualitatively and quantitatively; (2) to evaluate the differences between mitochondrial and cytoplasmic tRNAs in stability and structure in the presence and absence of pseudouridylation by PUS1p; (3) to study putative tissue specific differences in pseudouridylation by PUS1p; (4) to identify the interacting proteins and RNAs with PUS1p and measure their binding strength to normal and mutated PUS1p; and (5) to characterize the impact of the MLASA mutation on proteins and RNAs involved in iron metabolism. The health relatedness of this project is primarily (1) to provide guidance in the clinical management of patients with MLASA, and (2) to provide a more precise understanding of the clinical expression and pathophysiology of the inherited and acquired mitochondrial DNA disorders, as a first step to prevent and treat these common diseases.